PROJECT SUMMARY Gay and bisexual men (GBM), and other men who have sex with men (MSM), represent the largest group of individuals infected with HIV in the U.S. At current rates, 1 in 6 MSM will be diagnosed with HIV in their lifetime. Extant research has largely focused on identifying individual- and interpersonal-level risk factors for HIV infection among GBM; however, accumulating evidence also points to structural determinants of HIV outcomes in this population. In particular, recent research has demonstrated that structural stigma?defined as societal- level conditions, cultural norms, and institutional policies that constrain the resources and opportunities of stigmatized individuals?is associated with adverse health outcomes, including HIV risk, among GBM. For example, pilot data from our research group showed that 4,098 GBM living in states with high levels of structural stigma?measured with a composite index of discriminatory state laws and negative social attitudes towards homosexuality?had lower levels of post-exposure prophylaxis (PEP) and pre-exposure prophylaxis (PrEP) knowledge and use, lower odds of HIV testing, and greater odds of condomless anal sex compared with those living in low structural stigma states. While promising, the few existing studies of structural stigma and HIV outcomes among GBM have been limited by a reliance on cross-sectional designs, a small number of structural covariates, little attention to mediators/moderators of the structural stigma-HIV risk association, and non-probability samples of GBM. To address these limitations, we will create a new, prospective, nationally representative study of HIV-uninfected/unknown status GBM who will be followed for 5 waves of data collection over a 24-month period (N=500). With a representative sample of participants from across the U.S., we will have substantial variation in the social context surrounding GBM. This will be the first and largest prospective study with a nationally representative sample of GBM specifically designed to: (1) examine whether structural forms of stigma related to sexual identity increase vulnerability to adverse HIV prevention outcomes (e.g., less HIV testing and PEP/PrEP use); (2) identify mutable mechanisms (e.g., self-stigma, depressive symptoms, safer-sex self efficacy) through which structural stigma influences HIV outcomes; and (3) evaluate resilience processes (e.g., LGB community connectedness) that may mitigate the negative effects of structural stigma on HIV outcomes. In the final phase of the project, we will conduct qualitative interviews with 30 key informants and 30 GBM. We will use data from these interviews, together with the quantitative results, to inform the development of multi-level HIV intervention strategies that are maximally responsive to the structural conditions in which GBM are embedded. We have assembled a team of experts in structural stigma, HIV prevention among GBM, qualitative research, and survey research with nationally representative samples to address our study aims. This project stands to make important contributions to research on reducing disparities in the incidence of HIV infections among GBM, identified as a high NIH HIV/AIDS research priority.